Assemblies of electrochemical units connected in series (often called stacks) are known. The electrochemical units thus assembled may be formed for example by accumulator elements, or by fuel cells. A fuel cell is an electrochemical device for converting chemical energy directly into electrical energy. For example, one type of fuel cell includes an anode and a cathode between which a proton exchange membrane is arranged, often called a polymer electrolyte membrane. This type of membrane only allows protons to pass between the anode and the cathode of the fuel cell. At the anode, diatomic hydrogen undergoes a reaction to produce H+ ions which will pass through the polymer electrolyte membrane. The electrons generated by this reaction join the cathode by a circuit external to the fuel cell, thus generating an electric current.
Because a single fuel cell generally only produces a low voltage (around 1 Volt), fuel cells are often connected in series to form fuel cell stacks able to generate a higher voltage comprising the sum of the voltages of each cell. One drawback of fuel cell stacks is that disconnecting them is not sufficient to stop them. Indeed, if the current supplied at output by a fuel cell is suddenly reduced to zero, the fuel cells which form the stack are no longer able to eliminate the electrochemical energy they are producing, and the voltage at the terminals of the different cells is liable to rise to the point that it accelerates degradation of the polymer membrane and the catalysts associated therewith. It is not sufficient either to interrupt the supply of fuel and oxidant to stop a fuel cell stack. In this case, the quantity of fuel and oxidant enclosed within the stack is sufficient to maintain the reaction for a considerable period of time. In the case of a fuel cell stack that uses hydrogen as fuel and oxygen as oxidant, it may even take several hours for the stack to stop.
It is therefore advantageous to provide systems having electrochemical units, such as fuel cell stacks, with measuring devices for monitoring the voltage produced by each cell, so as to detect any variation when the system of electrochemical units is in constant operation or when it is in a stop phase.
There are known asynchronous measuring devices which take two forms.
In the first form illustrated in FIG. 1, electrochemical unit system 1 includes a plurality of differential amplifiers 4 each connected by two inputs to the terminals of an electrochemical unit 2, so as to supply a voltage representative of the potential difference between the terminals of said electrochemical unit 2. These differential amplifiers 4 are connected, at output, to a multiplexer 5, the output of which is connected to an analogue-digital converter 6. Multiplexer 5 is then operated to select each differential amplifier 4 in sequence, so that the voltage representative of the potential difference present between the terminals of said electrochemical unit 2 to which it is connected, can be sent to analogue-digital converter 6. This converter then digitizes said voltage and sends it to a processor or CPU 3, which recovers all of the digitized representative voltages in order, subsequently, to interpret them.
In the second form, illustrated in FIG. 2, electrochemical unit system 10 includes a first MUX1 and a second MUX2 multiplexer 13, wherein the positive terminal of each unit 11 is connected to the input of the first multiplexer 13 and the negative terminal of each unit 11 is connected to the input of the second multiplexer. The output of each multiplexer 13 is connected to an input of a differential amplifier 12. The voltage representative of the potential difference between the terminals of an electrochemical unit 11 is measured by selecting, via the first and second multiplexers 13, the potentials that correspond to said electrochemical unit 11. These potentials are sent to differential amplifier 12 in order to supply said voltage representative of the potential difference. The measurement of the voltage representative of the potential difference between the terminals of the next electrochemical unit 11 is then sent to an analogue-digital converter 15. The latter then digitizes said voltage and sends it to a processor or CPU 14, which recovers all of the digitized representative voltages in order, subsequently, to interpret this data.
The drawback of these two forms is that they are asynchronous. In fact, the process used in these two forms performs the measurements one after the other. Consequently, in the case of a stack of electrochemical units that includes around a hundred electrochemical units, the voltage of the hundred electrochemical units has to be measured in order to obtain the voltage representative of all the electrochemical units in the stack. Consequently, before the voltage of the first electrochemical unit can be measured again, the representative voltage has to be measured for all the electrochemical units. The time interval between two representative voltage measurements of the same electrochemical unit is thus too long.
Moreover, this method leads to a time lag between two voltage measurements of two contiguous electrochemical units. This means that it is not possible, at any determined time, to have the state of all of the electrochemical units, since the representative voltage of two contiguous electrochemical units has not been measured when the electrochemical units are in the same state, because a time lag has appeared. There therefore exists a risk that a voltage variation in one of the electrochemical units will not be detected and will cause damage to the system of electrochemical units. And even when a problem is detected, there is no way to tell which electrochemical unit is defective, since the voltage at the terminals of the units can vary according to operating conditions.